3GPP Long Term Evolution (3GPP LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology, such as UMTS (Universal Mobile Communications System), are currently deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on LTE called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (Rel-8 LTE). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN),” version 8.0.0, January 2009 (available at http://www.3gpp.org and incorporated herein by reference).
In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (UE). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in Rel. 8 LTE.
General Structure for Downlink Physical Channels
The general downlink LTE baseband signal processing according to 3GPP TS 36.211, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation”, version 8.6.0, March 2009, section 6.3 (available at http://www.3gpp.org and incorporated herein by reference) is exemplarily shown in FIG. 1. Further details on the LTE downlink can be found in 3GPP TS 36.211, section 6. A block of coded bits is first scrambled. Up to two codewords can be transmitted in one sub-frame.
In general, scrambling of coded bits helps to ensure that receiver-side decoding can fully utilize the processing gain provided by channel code. For each codeword, by applying different scrambling sequence for neighboring cells, the interfering signals are randomized, ensuring full utilization of the processing gain provided by the channel code. The scrambled bits are transformed to a block of complex modulation symbols using the data modulator for each codeword. The set of modulation schemes supported by LTE downlink includes QPSK, 16-QAM and 64-QAM corresponding to 2, 4 or 6 bits per modulation symbol.
Layer mapping and precoding are related to MIMO applications. The complex-valued modulation symbols for each of the code words to be transmitted are mapped onto one or several layers. LTE supports up to four transmit antennas. The antenna mapping can be configured in different ways to provide multi antenna schemes including transmit diversity, beam forming, and spatial multiplexing. Further the resource block mapper maps the symbols to be transmitted on each antenna to the resource elements on the set of resource blocks assigned by the scheduler for transmission. The selection of resource blocks depends on the channel quality information.
Downlink control signaling is carried out by three physical channels                PCFICH to indicate the number of OFDM symbols used for control in a sub-frame        PHICH which carries downlink ACK/NACK associated with UL data transmission        PDCCH which carries downlink scheduling assignments and uplink scheduling grants.Physical Downlink Control Channel (PDCCH) Assignment        
The physical downlink control channel carries scheduling assignments. Each scheduling grant is defined based on Control Channel Elements (CCE). The CCE corresponds to a set of resource elements. In 3GPP LTE, one CCE consists of nine Resource Element Groups (REGs). One REG consists of four Resource Elements (REs).
The PDCCH is transmitted on the first one to three OFDM symbols within a sub-frame. This control channel region consists of a set of CCEs, where the total number of CCEs in the control region of sub-frame is distributed throughout time and frequency control resource. Multiple CCEs can be combined to effectively reduce the coding rate of the control channel. CCEs are combined in a predetermined manner using a tree structure to achieve different coding rate.
In 3GPP LTE, a PDCCH can aggregate 1, 2, 4 or 8 CCEs. The number of CCEs available for control channel assignment is a function of several factors, including carrier bandwidth, number of transmit antennas, number of OFDM symbols used for control and the CCE size. Multiple PDCCH can be transmitted in a sub-frame.
On a transport channel level, the information transmitted via the PDCCH is also refereed as L1/L2 control signaling. L1/L2 control signaling is transmitted on the downlink for each UE. The control signaling is commonly multiplexed with the downlink (user) data in a sub-frame (assuming that the user allocation can change from sub-frame to sub-frame). Generally, it should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, then the L1/2 control signaling needs only be transmitted once per TTI.
Generally, the PDCCH information sent on the L1/L2 control signaling may be separated into the Shared Control Information (SCI) and Dedicated Control Information (DCI).
Shared Control Information (SCI)
Shared Control Information (SCI) carries so-called Cat 1 information. The SCI part of the L1/L2 control signaling contains information related to the resource allocation (indication). The SCI typically contains the following information:                User identity, indicating the user which is allocated        RB allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can be dynamic.        Duration of assignment (optional) if an assignment over multiple sub-frames (or TTIs) is possible        
Depending on the setup of other channels and the setup of the Dedicated Control Information (DCI), the SCI may additionally contain information such as ACK/NACK for uplink transmission, uplink scheduling information, information on the DCI (resource, MCS, etc.).
Dedicated Control Information (DCI)
Dedicated Control Information (DCI) carries the so-called Cat 2/3 information. The DCI part of the L1/L2 control signaling contains information related to the transmission format (Cat 2) of the data transmitted to a scheduled user indicated by Cat 1. Moreover, in case of application of (hybrid) ARQ it carries HARQ (Cat 3) information. The DCI needs only to be decoded by the user scheduled according to Cat 1. The DCI typically contains information on:                Cat 2: Modulation scheme, transport-block (payload) size (or coding rate), MIMO related information, etc. Note, either the transport-block (or payload size) or the code rate can be signaled. In any case these parameters can be calculated from each other by using the modulation scheme information and the resource information (number of allocated RBs).        Cat 3: HARQ related information, e.g. hybrid ARQ process number, redundancy version, retransmission sequence numberL1/L2 Control Signaling Information for Downlink Data Transmission        
Along with the downlink packet data transmission, L1/L2 control signaling is transmitted on a separate physical channel (PDCCH). This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the data is transmitted (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA). This information allows the UE (receiver) to identify the resources on which the data is transmitted.        The transport Format, which is used for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (receiver) to identify the information bit size, the modulation scheme and the code rate in order to start the demodulation, the de-rate-matching and the decoding process. In some cases the modulation scheme maybe signaled explicitly.        HARQ information:                    Process number: Allows the UE to identify the HARQ process on which the data is mapped.            Sequence number or new data indicator: Allows the UE to identify if the transmission is a new packet or a retransmitted packet.            Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version is used (required for de-rate-matching) and/or which modulation constellation version is used (required for demodulation)                        UE Identity (UE ID): Tells for which UE the L1/L2 control signaling is intended for. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.L1/L2 Control Signaling Information for Uplink Data Transmission        
To enable an uplink packet data transmission, L1/L2 control signaling is transmitted on the downlink (PDCCH) to tell the UE about the transmission details. This L1/L2 control signaling typically contains information on:                The physical channel resource(s) on which the UE should transmit the data (e.g. subcarriers or subcarrier blocks in case of OFDM, codes in case of CDMA).        The transport format, the UE should use for the transmission. This can be the transport block size of the data (payload size, information bits size), the MCS (Modulation and Coding Scheme) level, the Spectral Efficiency, the code rate, etc. This information (usually together with the resource allocation) allows the UE (transmitter) to pick the information bit size, the modulation scheme and the code rate in order to start the modulation, the rate-matching and the encoding process. In some cases the modulation scheme maybe signaled explicitly.        Hybrid ARQ information:                    Process number: Tells the UE from which hybrid ARQ process it should pick the data,            Sequence number or new data indicator: Tells the UE to transmit a new packet or to retransmit a packet.            Redundancy and/or constellation version: Tells the UE, which hybrid ARQ redundancy version to use (required for rate-matching) and/or which modulation constellation version to use (required for modulation).                        UE Identity (UE Tells which UE should transmit data. In typical implementations this information is used to mask the CRC of the L1/L2 control signaling in order to prevent other UEs to read this information.        
There are several different flavors how to exactly transmit the information pieces mentioned above. Moreover, the L1/L2 control information may also contain additional information or may omit some of the information. E.g.:                HARQ process number may not be needed in case of a synchronous HARQ protocol.        A redundancy and/or constellation version may not be needed if Chase Combining is used (always the same redundancy and/or constellation version) or if the sequence of redundancy and/or constellation versions is pre defined.        Power control information may be additionally included in the control signaling.        MIMO related control information, such as e.g. pre-coding, may be additionally included in the control signaling.        In case of multi-codeword MIMO transmission transport format and/or HARQ information for multiple code words may be included.        
For uplink resource assignments (for transmissions on the Physical Uplink Shared CHannel (PUSCH)) signaled on PDCCH in LTE, the L1/L2 control information does not contain a HARQ process number, since a synchronous HARQ protocol is employed for LTE uplink. The HARQ process to be used for an uplink transmission is given by the timing. Furthermore it should, be noted that the redundancy version (RV) information is jointly encoded with the transport format information, i.e. the RV info is embedded in the transport format (TF) field. The TF respectively modulation and coding scheme (MCS) field has for example a size of bits, which corresponds to 32 entries. 3 TF/MCS table entries are reserved for indicating RVs 1, 2 or 3. The remaining MCS table entries are used to signal the MCS level (TBS) implicitly indicating RV0.
For details on the TBS/RV signaling for uplink assignments on PDCCH please see 3GPP TS 36.213, “Evolved Universal Terrestrial Radio Access (E-UTRA); Physical layer procedures”, version 8.5.0, December 2008 (available at http://www.3gpp.org and incorporated herein by reference). The size of the CRC field of the PDCCH is 16 bits.
For downlink assignments (PDSCH) signaled on PDCCH in LTE the Redundancy Version (RV) is signaled separately in a two-bit field. Furthermore the modulation order information is jointly encoded with the transport format information. Similar to the uplink case there is 5 bit MCS field signaled on PDCCH. 3 of the entries are reserved to signal an explicit modulation order, providing no Transport format (Transport block) info. For the remaining 29 entries modulation order and Transport block size info are signaled.
Physical Uplink Control Channel (PUCCH)
The physical uplink control channel, carries uplink control information. UE never transmits simultaneously in the uplink control channel and uplink data channel. In case of simultaneous transmission of control and data, UE multiplexes control with data and transmits in the uplink data channel. The uplink control information could contain:                Uplink acknowledgement of decoded downlink transport blocks.        Channel quality information reporting for efficient downlink data transmissions.        Scheduling request for UL data transmission from UE to eNode B.Further Advancements for 3GPP LTE, 3GPP LTE-A        
The frequency spectrum for 1MT-Advanced was decided at the World Radiocommunication Conference 2007 (WRC-07) last November (Final Acts WRC-07, Geneva, November 2007). Although the overall frequency spectrum for IMT-Advanced was decided, the actual available frequency bandwidth is different according to each region or country. Following the decision on the available frequency spectrum outline, however, standardization of a radio interface started in the 3rd Generation Partnership Project (3GPP). At the 3GPP TSG RAN #39 meeting, the Study Item description on “Further Advancements for E-UTRA (LTE-Advanced)” was approved (see 3GPP TR 36.814, version 1.0.0, available at http://www.3gpp.org). The study item covers technology components to be considered for the evolution of E-UTRA, e.g. to fulfill the requirements on IMT-Advanced. Two major technology components which are currently under consideration for LTE-A are described in the following.
LTE-A Support of Wider Bandwidth
Carrier aggregation, where two or more component carriers are aggregated, is considered for LTE-Advanced in order to support wider transmission bandwidths e.g. up to 100 MHz and for spectrum aggregation.
A terminal may simultaneously receive or transmit one or multiple component carriers depending on its capabilities:                An LTE-Advanced terminal with reception and/or transmission capabilities for carrier aggregation can simultaneously receive and/or transmit on multiple component carriers.        An Rel-8 LTE terminal can receive and transmit on a single component carrier only, provided that the structure of the component carrier follows the Rel-8 specifications.        
It shall be possible to configure all component carriers LTE Release 8 compatible, at least when the aggregated numbers of component carriers in the UL and the DL are same. Consideration of non-backward-compatible configurations of LTE-A component carriers is not precluded
LTE-A Support of Relaying Functionality
Relaying is considered for LTE-Advanced as a tool to improve e.g. the coverage of high data rates, group mobility, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas.
The relay node is wirelessly connected to radio-access network via a donor cell. The connection can be                inband, in which case the network-to-relay link share the same band with direct network-to-UE links within the donor cell. Rel-8 UEs should be able to connect to the donor cell in this case.        outband, in which case the network-to-relay link does not operate in the same band as direct network-to-UE links within the donor cell        
With respect to the knowledge in the UE, relays can be classified into transparent, in which case the UE is not aware of whether or not it communicates with the network via the relay, and non-transparent, in which case the UE is aware of whether or not it is communicating with the network via the relay.
Depending on the relaying strategy, a relay may be part of the donor cell or control cells of its own.
In the case the relay is part of the donor cell, the relay does not have a cell identity of its own (but may still have a relay ID). At least part of the radio resource management (RRM) is controlled by the eNB to which the donor cell belongs, while parts of the RRM may be located in the relay. In this case, a relay should preferably support also Rel-8 LTE UEs. Smart repeaters, decode-and-forward relays and different types of Layer 2 relays are examples of this type of relaying.
In the case the relay is in control of cells of its own, the relay controls one or several cells and a unique physical-layer cell identity is provided in each of the cells controlled by the relay. The same RRM mechanisms are available and from a UE perspective there is no difference in accessing cells controlled by a relay and cells controlled by a “normal” Node B. The cells controlled by the relay should support also Rel-8 LTE UEs. Self-backhauling (Layer 3 relay) uses this type of relaying.
At least so-called “Type 1” relay nodes will be also introduced to of 3GPP LTE-A (see 3GPP TR 36.814, section 9.0). A “type 1” relay node (RN) is an inband relaying node characterized by the following properties:                The RN's control cells, each of which appears to a UE as a separate cell distinct from the donor cell.        The cells have their own Physical Cell ID and the relay node transmits its own synchronization channels, reference symbols, etc.        In the context of single-cell operation, the UE receives scheduling information and HARQ feedback directly from the relay node and sends its control channels (SR/CQI/ACK) to the relay node.        The RN appears as a Rel-8 Node B to Rel-8 UEs (i.e. will be backwards compatible).        
For 3GPP LTE-A UEs, it should be possible for a type 1 relay node to appear differently than Rel-8 Node B to allow for further performance enhancement.
LTE-A Support of Coordinated Multipoint Transmission/Reception Functionality
Coordinated multi-point transmission/reception is considered for 3GPP LTE-A as a tool to improve the coverage of high data rates, the cell-edge throughput and/or to increase system throughput (see 3GPP TR 36.814, section 8.0). Downlink coordinated multi-point transmission implies dynamic coordination among multiple geographically separated transmission points. Examples of coordinated transmission schemes include:                Coordinated scheduling and/or beamforming: Data to a single UE is instantaneously transmitted from one of the transmission points. Scheduling decisions are coordinated to control e.g. the interference generated in a set of coordinated cells.        Joint processing/transmission: Data to a single UE is simultaneously transmitted from multiple transmission points, e.g. to (coherently or non-coherently) improve the received signal quality and/or cancel actively interference for other UEs        
Downlink coordinated multi-point transmission should include the possibility of coordination between different cells. From a radio-interface perspective, there is no difference from the UE perspective if the cells belong to the same Node B or different Node Bs. If inter-Node B coordination is supported, information needs to be signaled between Node Bs. The following feedback and measurement mechanisms from the UE are supported:                Reporting of dynamic channel conditions between the multiple transmission points and the UE.        Reporting to facilitate the decision on the set of participating transmission points.        
Uplink coordinated multi-point reception implies reception of the transmitted signal at multiple, geographically separated points. Scheduling decisions can be coordinated among cells to control interference
PDCCH Coding for LTE-A
The PDCCH fields of the L1/L2 control information in 3GPP LTE, i.e. for a single component carrier assignment are exemplarily shown in the FIG. 2 for non-spatial multiplexing. In order to extend 3GPP LTE PDCCH to indicate multiple resource assignments on multiple component carriers for 3GPP LTE-A, different PDCCH coding schemes are being discussed in 3GPP RAN WG1 (see 3GPP TSG RAN WG1 Meeting #56, Tdoc. R1-090682, “PDCCH coding and mapping for carrier aggregation”, February 2009, available at http//www.3gpp.org and incorporated herein by reference). A brief outline of the PDCCH coding options discussed in this document is given in the following.
Uplink Acknowledgment Modes for 3GPP LTE-A
The uplink ACK/NACK transmission methods should be designed to support both symmetric and asymmetric carrier aggregation. The baseline assumption for downlink component carrier assignment is one transport block (in the absence of spatial multiplexing) and HARQ entity per scheduled component carrier. Thus in case of a multiple component carrier assignment, the UE may have multiple HARQ processes in parallel. This would mean that multiple ACK/NACKs corresponding to the downlink component carrier transport blocks should be transmitted in the uplink. This is unlike the case in 3GPP LTE Rel'8 where a single ACK/NACK report is transmitted in uplink. Further, it could be possible in 3GPP LTE-A that control and data channel are simultaneously transmitted within a single sub-frame To acknowledge the received downlink transport block(s) different uplink ACK/NACK transmission modes are presently considered in the discussions of the 3GPP.
Uplink ACK/NACK Non-Bundling Mode
In this mode ACK/NACK for each downlink component carrier transport block is sent by the UE. The transmission of multiple ACK/NACKs is generally preferable for UEs with no power limitation. Furthermore, separate ACK/NACK for each transport block results in the transport blocks being non-bundled or uncorrelated. This means that transport blocks can be (re-)transmitted independently of each other. Hence this allows for use of an individual HARQ process for each transport block.
Uplink ACK/NACK Bundling Mode
In this retransmission mode a single ACK/NACK is transmitted for multiple downlink component carrier assignments from one UE. UE generates a single ACK/NACK report comprising of one bit for non-spatial multiplexing or at least two bits for spatial-multiplexing mode by performing a logical AND operation per codeword across all component carrier assignments (A codeword is one modulation symbol obtained from modulating a coded transport block (systematic and parity bits)—typically there are multiple codewords for a transport block). This scheme is supported in LTE Rel'8 TDD and is considered to be beneficial in certain scenarios of 3GPP LTE-A. Since the UE transmits a single ACK/NACK report, less power consumption for UE and hence lower coverage loss.
Further, since a single ACK/NACK is sent for multiple component carrier assignments, the retransmission probability is high. This is not efficient from PDSCH HARQ process perspective. This scheme could however be beneficial for UEs in power limited conditions.
In a way, ACK/NACK bundling bundles the corresponding component carrier assignments (transport blocks). Bundling of transport block means that same transport blocks for re-transmission are considered as in initial transmission. Hence, this bundling of HARQ processes is not efficient, in case if the probability of ACK/NACK of the respective transport blocks is different.